This invention relates to an apparatus and a method for fabrication of optical waveguide Bragg gratings and the use of diffraction gratings in such fabrication.
Waveguide Bragg gratings are popular components used as wavelength selective filters in fiber-optic communication systems [1]. They are also popular for fiber-optic sensors because of their small size combined with the sensitivity of their reflection or transmission properties (typically the peak reflection wavelength) to strain, temperature and other mechanisms that change the fiber refractive index [2].
Fiber Bragg gratings (FBG) consist of a periodic modulation of the refractive index in the core (or more precisely within the modefield cross section) of an optical fiber [3]. The period of this modulation equals xcex9B=xcexB/2n where n is the fiber refractive index and xcexB is the optical Bragg wavelength at which the local reflectivity has its maximum. The index modulation is usually produced by illuminating the fiber core with ultraviolet (UV) light from a laser with wavelength xcexUV in the range from 190 to 300 nm and with a spatial intensity modulation period xcex9IF=xcex9B. Typically, a frequency doubled Argon ion laser with wavelength xcexUV≈244 nm may be used.
The intensity modulation can be created by splitting the laser beam and recombining the beams on the fiber via a mirror [4, 5] or lens [6] arrangement at an angle xcex1Fiber=2arc sin(xcexUV/xcex9IF). The UV beamsplitter can either be formed by a semitransparent mirror, a diffractive phase mask (spatially modulating the UV phase), or an amplitude mask (spatially modulating the UV amplitude). The angular separation between the +1 and xe2x88x921 order diffracted beams from such a mask equals xcex1Mask=2arc sin(xcexUV/xcex9Mask), where xcex9Mask is the mask period at the position where the mask is illuminated.
It is also possible to produce gratings by placing the fiber in the near field behind a diffractive mask where diffractive orders still overlap. However, there are disadvantages with the near field approach. First, the zero and higher order diffracted beams from available non-ideal masks will cause an unwanted background exposure which limits the available index modulation amplitude in a fiber with a limited photoinduced index change. Second, nonlinearities in the photosensitivity (index change versus UV exposure energy) at high exposure levels may induce background index variations due to the mixing of three or more diffractive orders. These background index changes will depend on the relative phases of the mixed beams within the fiber, and it will therefore depend on variations in the distance between the fiber and the phase mask in the order of xcex9IF. In practice it is very hard to avoid that the distance varies by many xcex9IF, hence the resulting background index variations will cause errors in the effective grating phase seen by light propagating along the fiber waveguide. A third problem arises if one wants to use scanning techniques where a small diameter UV beam is scanned across the fiber and mask to enable spatially dependent modulation of the grating phase and amplitude by moving and dithering the mask during the scan, as suggested in [7]. In this configuration (and also in the scanning fiber technique [8] discussed in more detail below) the fiber must be placed very close to the mask (typically at a distance of 100-300 xcexcm) to ensure overlap between the +1 and xe2x88x921 order diffracted beams. The need for such a small spacing induces a high risk of damaging the delicate mask surface, as well as potential problems with static electric forces between the fiber and the mask, as discussed in [6].
Various stepwise grating exposure techniques have been disclosed. Two similar systems are disclosed in [8, 9], where the fiber is moved through a stationary UV interference pattern in the direction parallel to the fiber axis while short pulses are fired from the UV source at regular intervals when the phase of the UV intensity pattern relative to the fiber matches the wanted index modulation phase. A disadvantage of this pulsed technique is that the duty-cycle of the pulsing UV source must be low, preferably  less than 20-30% in [8] and even lower in [9], to enable high visibility of the exposure and thus low background index change. With limited peak UV intensity available, for instance due to limited source power available or to damage limitations of the fiber or the UV optics, this will cause significantly longer production times compared to approaches using continuous exposure. Short writing time per FBG is generally advantageous, since it minimizes grating errors caused by slow drift in the relative positions of components in the FBG production system.
An alternative version of the step writing technique where the UV source is operated in a continuous wave mode is disclosed in [4]. In this approach the interference fringes are moved at the same speed as the fiber by moving the diffractive mask, but the mask position is reset each time the fiber has moved some multiple of Bragg periods. This technique can allow for shorter writing times than the techniques described in [8, 9].
A potential problem with the step writing techniques is that the periodic pulsing of the UV source or the resetting of the mask will tend to cause a super periodic modulation of the grating strength and/or phase with a super period length that corresponds to the pulse or resetting period. If the resetting period equals MLB where M is an integer, this will cause grating sideband reflections at wavelengths that correspond to Bragg wave numbers which are separated from the nominal Bragg wave number 2p/LB by integers of 2p/(MLB). There are two reasons why M preferably should be a small number. First, the sideband separation can usually be made so large by increasing M that the sidebands do not cause any problem for the FBG applications of interest. Second, as discussed in [9] the strength of the sideband reflections will be reduced when M is reduced due to averaging. This is because the number of step exposures at each position on the fiber is increased.
In the mask stepping approach, maximizing the speed and accuracy by which the mask position is reset will also contribute to minimize the super periodic modulation and the grating sideband reflectivity. In order to minimize M and to maximize the resetting speed it is thus desirable that the fringe position can be modulated with a large bandwidth. This may limit the size of the mask, as the mechanical resonance frequencies are typically inversely proportional to the size of the modulated device (the mask).
It is possible with the step writing techniques to impose discrete phase-shifts in the index modulation by suddenly changing the phase of the interference fringes as the fiber is scanned through the UV spot. The fiber section that is illuminated by the UV spot when the fringe phase changes will in this case be partially exposed with both fringe phases, and the grating phase-shift will thus occur gradually across this section. Apodization of the grating strength can be implemented without perturbing the grating phase or the mean exposure per grating period by modulating the phase of the fringes relative to the fiber while the fiber is scanned through the UV spot, without perturbing the mean phase value.
It is also possible to stretch or compress the Bragg period xcex9B slightly relative to the UV interference period xcex9IF=xcexUV sin(xcex1Fiber/2) by gradually changing the fringe phase relative to the fiber as the UV spot is scanned along the fiber. The Bragg wavelength shift achievable by this method is limited to about xcex94xcexB,max=xcex9IF2/(4 LSpot) where LSpot is the 3 dB UV interference spot size [5]. When the shift increases above this limit the visibility of the integrated exposure decreases substantially because the fringe phase exposed to each fiber position varies significantly when the fiber moves through the UV spot. With typical numbers LSpot=0.1 mm, xcexB=1550 nm, and n=1.465 we have xcex94xcexB,max=xcexB2/(4nLSpot)=2.1 nm. Gratings covering bandwidths larger than 2xcex94xcexB,max cannot be exposed in a single scan with the prior art step writing techniques.
Multi-channel gratings [10] are promising devices both for telecommunication and sensor applications employing wavelength division multiplexing (WDM), as they comprise the functionality of a number of single gratings into one grating making a more compact device. The index modulation of a multi-channel grating with N Bragg wavelength can be modelled as       Δ    ⁢          xe2x80x83        ⁢    n    =      Δ    ⁢          xe2x80x83        ⁢          n      0        ⁢                  ∑                  i          =          1                N            ⁢              cos        ⁢                  xe2x80x83                ⁢        2        ⁢                  π          ⁡                      (                                          1                /                                  Λ                  B0                                            +                              1                /                                  Λ                  P                                            +                              φ                i                                      )                              
where xcex94n0 is the index modulation amplitude of each subgrating, 2xcfx80/xcex9BO is the Bragg wavenumber offset, 2xcfx80/xcex9P is the Bragg wavenumber channel spacing, and xcfx86i is the phase of sub-grating i. xcex94n is periodic with period xcex9P. If the phases xcfx86i of all subgratings are equal, the peak-to-peak index modulation will equal xcex94npp=2Nxcex94n0, as illustrated by curve C in FIG. 1 for the case N=16. As the available index change in photosensitive fibers is limited, it may be advantageous to minimize xc3x97npp. For large values of N the value of xcex94npp can be reduced to the order of 2{square root over (N)}xcex94n0 by carefully selecting the phases xcfx86i as shown in [11]. Numerical calculations indicate that xcex94npp can be reduced to (2{square root over (N)}+1.5)xcex94n0 or less for N greater than 4, as illustrated for N=16 by curve A in FIG. 1. It is therefore of interest to find a method for writing multi-channel gratings where the subgrating phase relationship can be accurately controlled. Also in multi-wavelength fiber Bragg lasers, proper control with the relative phase the grating channels is of importance [12].
The step-writing method described above allows arbitrary multi-channel gratings to be produced in a single writing operation. However, the total number of channels must be confined within the available bandwidth of 2xcex94xcexB,max. It is also possible to write multi-channel gratings sequentially, inscribing one channel at the time. If the total bandwidth covered by the channels exceeds 2xcex94xcexB,max the UV interference period xcex9IF needs to be changed between each writing cycle. This can be accomplished by replacing the diffractive mask and/or tuning the interferometer mirror or lens positions, possibly combined with a tilting of the mirror axis. The distance between the fiber and the diffractive mask may also have to be adjusted to ensure proper overlap of the interfering beams at the fiber. In this replacement and readjustment process the task of maintaining control with the relative phases of the different subgrating exposures is very difficult.
A method will be described in the following for ensuring control of the relative phase of the different subgratings during such sequential exposures. However, each writing cycle will always contribute with a mean index of xcex94n0, and the total index modulation including background index will therefore at least be in the order of (N+{square root over (N)}+0.75)xcex94n0, as illustrated by curve B in FIG. 1. The only way to avoid a background index in the order of Nn0 is by inscribing all Bragg wavenumbers grating in a single exposure.
If the step-writing methods described above are used to produce multi-channel gratings covering a large bandwidth ( greater than 20 nm), the UV spot size LSpot would need to be extremely small (at least  less than 20 xcexcm). It is possible to focus UV laser beams extremely tight. However, because the UV intensity must not exceed the damage threshold of the fiber, the time required to write a single grating may become unacceptably long.
It is a first objective of the present invention to provide an easy and accurate method and a device for adjusting the interference period xcex9B in an optical waveguide Bragg grating production facility over a wide range, thus allowing the facility to easily produce a wide range of Bragg wavelengths in a flexible way.
A second objective is to provide a method and device for varying xcex9B during the grating exposure while maintaining accurate control with the phase of the exposed pattern during the exposure.
A third objective is to provide a practical method and device for writing large bandwidth multiple channel optical waveguide gratings with high spatial resolution superperiodically modulated grating profile with (a close to) optimum utilization of the available refractive index change, which does not require focusing of the laser beam down to a spot size that is comparable to or smaller than the wanted spatial resolution.
A fourth objective is to provide a practical method and device for writing multiple channel optical waveguide gratings with high spatial resolution superperiodically modulated grating profile, which only requires a diffractive mask that is slightly longer than one superperiod length plus the UV interference spot size.
A fifth objective is to provide a fast and effective method for writing waveguide gratings with a step exposure technique with a short stepping period, which maximizes the strength of the grating and minimizes the amplitude of unwanted grating, which does not require high bandwidth modulation of the phase mask position, or put limitations on the size of the phase mask that can be used.
The objectives are met with an apparatus, methods and uses according to the attached independent claims.